This invention relates to cardiac pacers and relates more particularly to an improved synchronous ventricular programmed pacer.
Existing cardiac pacers are generally of two basic types. The first type, designated fixed rate pacers, provides a constant rate output independent of biologic cardiac activity. This type of pacer is simple, and therefore quite reliable, but can be unsafe if normal cardiac activity returns unexpectedly. In an attempt to overcome this problem, a second variety of more complex R wave programmed pacers has been developed. These units attempt to overcome the principal disadvantage inherent in fixed rate pacers by providing means to monitor heart activity and control pacer output as a function thereof to prevent competition between pacer stimuli and normal cardiac activity.
Prior art programmed pacers, also referred to as adaptive or triggered pacers, can be divided into two basic categories. The first category is the demand pacer, in which the generation of artificial cardiac stimuli is inhibited during normal heart function. Pacers in this category are also referred to as inhibited or suppressed type units. The second category of programmed pacers, referred to as R wave synchronous, triggered or standby pacers, are not inhibited during normal heart function but, rather, emit pulses synchronized with normal heart activity. In the absence of normal heart activity, both categories supply stimuli at a predetermined fixed rate, usually in the order of 70 pulses per minute.
Each of the two known categories of programmed pacers have certain inherent deficiencies. Demand pacers are designed to deliver fixed rate pacing pulses whenever the natural heart rate falls below a predetermined minimum. This type of unit is therefore designed to shut itself off whenever the sensed heart rate is above the predetermined rate. Accordingly, a detected interference signal occurring at normal heart signature rate will completely inhibit the operation of a demand pacer, resulting in complete loss of pacer function. Interfering signals at pulse rates below normal heart rates can also be a serious problem in this type of unit since these rates, when combined with the low physiologic rate encountered in heart block, can also cause complete loss of pacer function. Attempts to filter or otherwise attenuate interference signals have not been fully successful.
Synchronous R wave pacers are designed to overcome this problem by permitting output pulses to occur in synchronism with sensed heart rates during normal cardiac activity rather than being inhibited by such activity as are demand pacers. Should the normal heart rate fall below a predetermined minimum rate, for example 70 pulses per minute, the synchronous pacer delivers pacing pulses at a fixed, predetermined rate that overrides the lower physiologic activity rate. Accordingly, synchronous pacers are not inhibited by external interference fields at biologic rates, as are demand pacers.
In overcoming the interference problem, however, conventional synchronous pacers introduce a second serious problem. In the conventional synchronous pacer, it is necessary to incorporate an input inhibit period, referred to as the refractory delay period, during which the synchronous pacer is blind to input activity from the heart as well as from interference sources. The purpose of this refractory delay period is to prevent the synchronous pacer from triggering on interference signals at higher than biologic rates. Thus, each time the synchronous pacer delivers an output pulse to the heart, the unit becomes insensitive to either a noise input pulse or a premature ventricular contraction (PVC) occuring during the input refractory delay period. While it is desirable to thus make the unit insensitive to noise pulses during the refractory period, the inability to recognize a PVC may result in the discharge of a competitive artificial stimulus during the vulnerable period which follows the PVC. This condition has the potential for inducing fibrillation.
One prior art solution to this latter problem is to reduce the input refractory delay period to minimize the period of insensitivity to PVCs. However, this solution leads to other disadvantages in the presence of relatively high rate interference fields. Since the output rate of the synchronous pacer in noise fields is limited only by the duration of the input refractory delay period, as this period is shortened to improve recognition of PVCs, the maximum output rate of the unit in higher frequency noise fields can increase beyond the maximum permissible pacing rate. Thus, a conflict exists between the necessity for minimizing the refractory period to minimize the possibility of not detecting PVCs and the necessity for increasing the refractory period in order to ensure a safe maximum rate in noise fields. In conventional synchronous pacers, a typical compromise value for the input refractory delay period is about 400 milliseconds, which allows a maximum rate of about 150 pulses per minute in the presence of interference at or above this rate. This compromise is far from ideal, since a heart rate of 150 beats per minute can decompensate elderly patients, or those with cardiac or vascular disorders. Furthermore, the 400 millisecond refractory delay still permits some early PVCs to remain unsensed, so that the possibility of competitive pacing is not eliminated.
Representative prior art demand pacers are shown in U.S. Pat. Nos. 3,661,157, 3,678,937 and 3,693,626. These units are all subject to being inhibited in the presence of external interference at rates which mock normal biologic heart rates. A prior art synchronous pacemaker is shown in U.S. Pat. No. 3,433,228. This unit, in common with other prior art synchronous pacers, cannot sense premature ventricular contractions that fall within its own refractory delay period, and is also capable of being triggered at relatively high rates by interference. A complex dual channel pacer is disclosed in Fischler et al Atrial Synchronized Demand Heart Pacing, IEEE Transactions on Biomedical Engineering, Vol. BME-16, No. 1, January 1969. The disclosed system is based on both synchronized and demand pacemaker concepts, but is nevertheless subject to being completely inhibited in the presence of interference fields in the range of normal heart rates.